How to export an Image(png / jpg) as glTF?
The imported image is using the "Diffuse BSDF" shader, but glTF has a PBR material system that is based on the "Principled BSDF" shader.

Luckily, you can just swap one for the other to get this working.  Use the "Shading" tab at the top of Blender 2.80 to get to this area, and replace "Diffuse BSDF" with "Principled BSDF":



Additionally, if your image has any translucent parts, set "Blend Mode" to "Alpha Blend" in the material settings (Eevee only) like so: